Well control method



March 3, 1970 A, G. WEST ETAL WELL CONTROL METHOD Filed sept. 26, 1967133 lil l J4 35 J5 i .Z

[Nl/ EN TOR 5 United States Patent O ice 3,498,393 WELL CONTROL METHODAlfred Gordon West, Chester M. Harden, and Clyde E.

Pearce, Midland, Tex., assignors to W&H Production- Drilling, Inc.,Midland, Tex., a corporation of Texas Filed Sept. 26, 1967, Ser. No.670,559 lint. Cl. EZlb 21/04; E21c 7/08 US. Cl. 175-48 2 Claims ABSTRACTOF THE DISCLOSURE A method is provided for preventing blowouts duringdrilling of an oil or gas well, which includes circ-ulating a drillingfluid down the borehole, the drilling fluid absorbing gas from thestrata being cut by the drill bit. The returning drilling Huid is passedinto a separator wherein the gas is separated from the drilling fluid.The ow rate of the gas is continuously measured and from the results ofthe llow rate measurements the porosity of the formation and bottom holepressure of the well are indicated. Since it is generally necessary tokeep the hydrostatic head somewhat greater than the bottom holepressure, the hydrostatic head is then adjusted by increasing ordecreasing the density of the drilling fluid.

Apparatus is provided for measuring the gas flow rate from the separatorwhich includes a plurality of parallel pipes of differing sizes. Eachpipe has a flow rate detector, and suitable valves are included forpassing the gas flow into whichever pipe is desired, depending on theflow rate. If the flow rate is great, a large pipe is used; if the flowrate is small, a small pipe is used.

BACKGROUND OF THE INVENTION One of the most critical problems indrilling an oil or gas well into the earth is the tendency to losepressure equilibrium under certain well conditions, resulting in blowoutof the well. A blowout is the wasteful blowing of oil and gas out of thewell and the complete loss of control of pressure. In view of thetremendous capital investment necessary to drill a well (drilling costsoften run to many hundreds of dollars per day and the drilling has oftenbeen proceeding for many days when the greatest risk of blowout occurs)it is seen that a blowout is almost always an enormously expensivething. In addition to the expense involved, a blowout is one of the mostdevastating and destructive things that can happen to a` well; wellswhich have blown out are almost always damaged (damage which may berealized throughout the life of the well), and often must be completelyabandoned. Even if the well can be brought under control, a great dealof time is often lost in drilling and special equipment and extra labor(which may not be readily available) are needed to bring the well undercontrol.

A still further hazard of the blowout is that the friction of theequipment at the well head can be sufcient to cause a re in the blowoutgas, and of course such a re is not only extremely expensive but verydangerous to the workmen at the rig, and ruinous to the rig equipment.

Blowouts most often occur in formations which contain high pressure gaspockets. In the drilling process a drilling fluid or drilling mud iscustomarily circulated down the borehole to clean the hole of cuttingsand lubricate the drill bit. Blowouts can be prevented by establishingpressure control with this drilling mud. That is, if the pressure of thehydrostatic head of the drilling mud is at all times kept greater thanthe bottom hole pressure of the well, blowouts may be prevented. And thehydrostatic head of the drilling mud can be varied by varying thedensity of the mud, as is well known in the drilling 3,498,393 PatentedMar. 3, 1970 art. Further, the normal formation pressure of the stratabeing penetrated may be determined by standard methods. Consequently, itis the present practice to maintain the hydrostatic head at a levelconsiderably greater by some safety factor than the normal bottom holepressure.

There are two grave difficulties with the use of this safety factortechnique. The first is that it does not compensate for the abnormalconditions which sometimes develop, and consequently is only partiallyeffective in preventing blowouts. Secondly, in using the prior artmethods a relatively large differential between hydrostatic head andbottom hole pressure must be maintained, which means that thehydrostatic head is at nearly all times much greater than is reallynecessary.

Maintenance of the hydrostatic head of the drilling uid at a level abovewhat is necessary is in itself a signicant problem to the driller. Forexample, it has been shown that a lighter hydrostatic head results in afaster penetration rate. And of course, a faster penetration rate isdesirable in drilling of wells because of the savings in time requiredto drill a Well (and hence the cost of drilling), realized. Further, ahigher hydrostatic head will result in injury to certain formationswhich are low in pressure. That is, if the pressure in the bore hole issignicantly greater than the pressure in an adjacent formation, theformation might be fractured by the drilling fluid in the borehole,thereby permanently injuring the formation and forever damaging itspermeability to thereby result in permanent decrease of the productivityof the well. Such fracturing is also undesirable because a great deal ofthe drilling fluid can be lost into the fractured formation. It has alsobeen that longer bit life is achieved with a lighter hydrostatic head.That is, the bit will not wear out so quickly if the mud weight isdecreased. This is extremely important since when the bit does wear out,the drilling must be stopped and a trip must be made to replace the bit.This can often take many hours of valuable downtime on the rig. It alsoresults in a buildup of trip gas during the trip, and this gas furthercontributes to the blowout problem.

As hazardous and as expensive of a problem as it is, a blowout does notoccur instantaneously. Rather, it is something which builds up over aperiod of time (a time period which may be shorty however, if theformation permeability is great) and then, perhaps, is ratherinstantaneous in its result. For this reason blowouts can be preventedand continuous pressure control established by the use of the presentinvention.

SUMMARY OF THE INVENTION The present invention provides a method formaintaining pressure control while drilling a well such as an oil or gaswell, to thereby prevent the occurrence of a blowout. The methodincludes generally pumping a drilling fluid such as drilling mud into aborehole while drilling to establish circulation in such a manner thatmud introduced into the borehole at the surface proceeds to the bottomof the borehole and then back to the surface. In this manner, thereturning mud contains the gases which are present in the borehole. Uponits return to the surface, the mud is introduced into a separator andthese gases are sepa-rated from the mud. The mud may be returned to amud pit for further use.

The gas from the separator is then passed through appropriate lineswherein instruments are located which measure the volume of gas iiow. Inthis connection, lines of various sizes may be constructed in parallelso that the gas may be directed through different size lines dependingon the amount of gas being produced, for more precise measurement. Themeasurements are recorded, and a record is kept of the measurements fromhour to hour and from day to day.

These flow rate measurements will indicate to the operator whether gasis being depleted (smaller flow rate) or whether additional gas is beingencountered (greater flow rate). Thus, the well operator is apprised ofany unusual condition in sufficient time to take whatever remedialaction is necessary. It is noted that the goal sought to be achievedwith the use of this invention is the operation of the drilling methodwith the least possible hydrostatic head on the drilling fluid.

This hydrostatic head uid may be adjusted when necessary by merelyincreasing or decreasing the density of the drilling mud. If the gaspressure is increasing, the density of the mud is increased to preventblowout; if the. gas pressure is decreasing, the density of the mud isdecreased in order to effect longer bit life, better penetration rate,and to prevent fracture of vulnerable formations.

Pressure control is thus constantly effected during drilling of the wellwith the result that the possibility of blowout is minimized and thebenefits of lowest possible hydrostatic head are realized.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE l is a schematic elevationalview, partially in section, of a borehole during the drilling process,wherein the method of the present invention is employed; and

FIGURE 2 is a side view of apparatus according to one embodiment of thisinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS A schematicillustration of a well drilling operation employing the presentinvention may be seen in FIG- URE 1.

Here it is seen that a borehole has been drilled into the earthextending from the surface 12 to a point 14 beneath the surface, througha plurality of different geological formations 16, 18, and 22. It willbe readily understood that as drilling proceeds the borehole 10 will getdeeper and deeper.

The borehole 10 is being drilled in this instance by a rotary drill bit24 suspended at the end of a string of drill pipe or a drill string 26.The bit 24 is rotated by suitable power means 34 located on the derrickfloor 36 at the earths surface. Suitable drill collars 28 may bepositioned on the drill string just above the drill bit 24 in order toweight the bit and cause it to more effectively drill the formation.

The borehole wall may be protected by the installation of a casing 30 toany desired level in the borehole. In those regions protected by thecasing 30, the problem of fracturing discussed above is overcome.

A drilling fluid which may be water or oil, but is commonly mud (amixture of clay and water with suitable additives), is circulated duringdrilling from the surface down the center 32 of the hollow drill string26, past the drill bit 24 at the bottom 14 of the hole, and thence upthe annualar area 33 outside the drill string between the drill stringand the casing 30 or borehole wall. As the bit 24 is rotated with thedrill string by the power means 34, it cuts away at the adjacentformation 22, forming loose cuttings at the bottom of the hole. As thedrilling mud sweeps the bottom of the hole, it carries these cuttingsupwardly through the borehole to the earths surface. The drilling mudalso cools the bit, and serves to maintain the hole gauge by itscontinual sweeping action.

But the drilling mud provides another vital function and that is toserve as a balance against the bottom hole pressure to thereby preventblowout. The hydrostatic head is directly related to the depth of theborehole and the density of the drilling mud, by the following equation:

where p is the hydrostatic head in p.s.i., d is the density of the mudin pounds per gallon, and lz is the depth of the column in feet.

Mud density may be regulated by conventional and well known methods(literature available from National Lead Company, Baroid Division, forexample) by the addition of weighting additives to the mud before it isintroduced into the borehole, and may be readily determined by suitablemeasurements well known to those of skill in the art. Therefore, it is arelatively simple matter to calculate the hydrostatic head and to adjustthe hydrostatic head to any desired value at all times.

The pressure of the fluid in the pores of a rock formation determinesthe formation pressure of that particular strata. The various formations16, 18, 20 and 22 through which the borehole penetrates exist atdifferent formation pressures. Since the normal pressure gradient isknown for the particular area of drilling, the normal formation pressuremay be calculated simply by multiplying the normal gradient times thedepth of the borehole. To balance this formation pressure, the drilleradjusts the hydrostatic head by increasing or decreasing the density ofthe drilling mud as mentioned above. And then to allow for pressuresurges and higher than expected pressures, a safety factor is added;that is, the hydrostatic head is maintained at a value considerablygreater (for example, 750 p.s.i. greater) than the normal formationpressure. This safety factor is necessary because, as mentioned above,the formation pressure varies even within one formation; further, thedrill may suddenly hit a relatively high pressure gas pocket.

There are at least two principal difficulties with the use of thesesafety factors in present practice, as discussed above. The first andmost critical is that they are sometimes not large enough to compensatefor the extremely high-pressure gas pockets which are sometimesencountered, especially in certain areas of Texas and Louisiana. If sucha gas pocket is encountered and the gas pocket formation is of suicentpermeability, a blowout may occur. The second problem with the safetyfactor practice is that the safety factors used are (for the greatmajority of situations) overly large, to compensate for problems such asthat just described. Use of these large safety factors means of coursethat the hydrostatic head is maintained at a level significantly aboveand beyond what is generally needed, resulting in lower bit penetrationrate, shorter bit life, consequent increase in cost of drilling thewell, and the risk of fracturing a low-pressure formation.

In addition to these difficulties, when it is determined that ahigh-pressure pocket has been encountered (and a blowout has notoccurred), drilling through such pocket under present practice is slowedto a very slow pace, resulting again in increased cost of drilling.

Some formations are known as high-pressure, lowvolume formations.Because of their low porosity, there is no real blowout danger in someof these formations even though the gas exists in the pores of thestrata at high-pressure. This is simply because not enough of thehigh-pressure gas can nd its way into the borehole. These formations areoften of no real concern to the driller, as far as the blowout problemis concerned, so in some formations only, the ybottom hole pressure maysafely be greater than the hydrostatic head.

As the drilling mud sweeps the bottom of the borehole, as seen in FIGUREl, gas from the formation being cut by the drill bit will enter theborehole 10 and will be returned to the surface up the annulus 33 by themud. Some relatively small amount of the gas will be dissolved in themud. In accordance with this invention, when this gascontaining mudreaches the surface it is transported by suitable means such as the pipe38 to a separator 40 suitable for separating the gas from the mud.Suitable choke means may be included in the line 38 to reduce thepressure of the mud-gas mixture to a level which may be properly handledby the separator. In this connection, it has been found particularlyadvantageous to use the two-stage system illustrated in FIGURE l,comprising the adjustable choke 84 and the hydraulic choke 86. Thetwostage pressure drop effected in this manner has been found to beeffective in preventing high gas pressures from cutting the equipment.

The separator 40, which may operate at any convenient pressure as forexample 125 p.s.i., may be elevated so that the mud may return bygravity through a mud return line 44 to a mud pit 46, for laterrecirculation into the well.

The gas exits the separator 40 through a gas line 42, and is thencemeasured to determine the volume of gas released from the separator.Such measurement may be made in any suitable manner, one example beingillustrated in FIGURE 2.

A multiple ow line system which has been fonud to be particularlyadvantageous for the measurement of gas ow under all conditions is shownin FIGURE 2. The primary advantage of this particular system is that itallows accurate and precise flow measurements for gas ow rates rangingfrom very high to very low. In accordance with this system, severallines of different sizes are connected in parallel, with instruments formeasuring flow rate positioned in each line. Since it is dicult to getaccurate and precise measurements when there is very little flow througha large line, or when there is large flow through a small line, the owis directed by the operator through the appropriate line by suitable owcontrol means, depending on the volume of ow.

In this embodiment, the line 42 from the separator may ybe six inches indiameter. Parallel line 50 may be four inches in diameter and may besecured by an elbow connector 51 to the line 42 at a point spaced fromthe separator 40. Another parallel line 54, which may be three inches indiameter, may be secured by appropriate elbow connection 55, to the line50.

Each of the parallel lines is in fluid communication at a point spacedfrom their connection with each other, with a transverse line 62 whichcommunicates with a flare 64 whereby the waste gas is burned a safedistance from the r1g.

Suitable flow control means such as valves are located at appropriatepositions in the lines, as seen in FIGURE 2. Valve 52 is located in line42 downstream from the connection of line 42 with line 50. Valve 58 islocated in line 50, downstream from its connection with line 54. Intransverse line 62, suitable valves 68, 70, and 72 are located betweenline 42 and flare 64, line 42 and line 50, and line 50 and line 54,respectively. A small line 60 for use with very small ow rates may belocated transverse to the parallel lines 42, 50, 54 and secured in uidcommunication between the lines 54 and 42. This line may be for example3A inch in diameter, and may contain an orifice 66 of any suitable sizetherein.

A rupture plate 56 which may be adapted for rupture when the pressurereaches a certain level, for example `100 p.s.i., is located in line 54near its connection with line 50.

Suitable ow measurement means, such as pitot tubes, are located in thevarious lines. Tube 74 is positioned in line 42 between the separator 40and the connection with line 50; tube 76 is positioned in line 50between its connections with the lines 42 and 54, tube 78 in line 54between its connections with the lines 50 and 60, and tube 80 in theline 60 between its connections with the lines 54 and 42, preferably atthe orice 66. Each of these tubes or other flow indicators is suitablefor measuring the flow rate of the gas passing through the line whereinit is located, and suitable means are connected with each such tube forcontinuously recording the data received on a recorder 82. In thismanner, a continual record of the flow rate of gas returning from thewell may be kept.

The manipulation of the various valves in order to effect flow throughany of the desired lines is readily apparent yby reference to FIGURE 2.Thus, when the ow rate is relatively large and ow is desired through theline 42, the valve 52 is opened and the valve 58 is closed. But if theow through line 50 is desired, then the valve 52 is closed and the valve58 is opened.

Since the flow rate of the gas released from the separator is, assumingconstant permeability directly proportional to the bottom hole pressurein the well, these flow measurements enable the driller to keep aconstant, up-to-the-minute account of what is happening, pressurewise,downhole. And -by comparing the flow rate measurements with similarmeasurements taken earlier, the rate of change in the flow rate may alsobe determined. These measurements thus allow the driller to determinewhether a gas zone is depleting or whether additional gas is beingencountered, and thus to predict what is happening downhole at any givenmoment. The operator knows when the drilling has proceeded into ahigh-pressure gas pocket. The hydrostatic head is then adjusted whennecessary by decreasing or increasing the density of the drilling mud bymixing with the mud the appropriate additives.

It is still necessary, of course, in formations which are not relativelylow volume formations, to maintain the hydrostatic head at a levelsomewhat greater than the bottom hole pressure. It will :be recognizedthat the dilerential employed will vary somewhat depending on theformations being drilled and other factors, but in many contexts of use,it is found that a pressure dierential of about 200-250` p.s.i. issufficient. This is considerably less than the differential necessaryfor safety with conventional drilling processes. And it is thedifference between the hydrostatic head levels which may be utilized Asan example, if it is determined that a 250 p.s.i. differential isappropriate for use in a given situation, the mud density is continuallyadjusted so that the hydrostatic head is maintained at a level exceedingthe bottom hole pressure by 250 p.s.i. Complete pressure control is thusmaintained without danger of blowout.

When a high-pressure gas pocket is struck, the driller will have timeusing the method of the present invention to increase the mud densitysuiciently to avoid blowout. This is because there is a delay or lagtime between the Ibits striking the gas pocket, and any possibleblowout, because formations are not suiciently permeable to causeinstantaneous blowout. Of course, this lag time varies greatly dependingon the permeability of the formation and the pressure of the gas pocket,and therefore it is greatly desired that fairly immediate correctiveaction be taken whenever necessary. This of course is one of the greatadvantages of the present invention.

It is here noted that there are presently methods for determiningbottom-hole pressure, such as that known as the drill stem test. Butsuch methods are expensive, dangerous, and cannot be continuouslyperformed during drilling of the well. If it is desired to take such atest during drilling of the well while the process of this invention isbeing used, then such a spot check will allow the driller to bettercorrelate the data received. For example, if the ow rate is increasing,the driller can tell just exactly how much of the increase is due to anincrease in bottom hole pressure and how much is due to increasedporosity of the formation. Such proced-ures are not generally necessaryhowever. Since the formation porosity is generally known at least tosome degree, the measured flow rate of the gas returning from the well(and the rate of change therein) has been found to give a direct andaccurate indication of the bottom hole pressure picture. In other words,although the porosity of the formation does have an efect on the ow ratemeasurements, such effect can usually be discounted to great extentbecause of the knowledge the driller will 7 have of the formation beingdrilled, and the safety factor allowed.

It is seen that the invention has provided a method for well controlwhich provides for complete pressure control at all times with minimumrisk of costly and dangerous blowout. Further, the invention provides amethod which allows for faster penetration rates and longer bit life,resulting in reduced drilling costs. The method of the present inventionalso is seen to allow for faster drilling through strata wherein theformation pressure is high, and to provide protection against thefracture of low pressure formations.

It is further seen that the present invention provides apparatussuitable for accurately and precisely measuring the llow rate of gasemanating from the well, regardless of whether the ilow is great orsmall. The apparatus is desirably portarble so that it may be readilytransported from rig to rig.

What is claimed is:

1. A drilling method which enables the driller to maintain drilling witha low hydrostatic head and at the same time prevent blowout whiledrilling a borehole for an oil or gas well into the earth throughgas-containing formations, whereupon gas from said formations enters theborehole, comprising:

passing drilling iluid down into said borehole and circulating saidiluid back to the earths surface, whereupon gas in the borehole from thesurrounding formations is returned to the earths surface by the drillingfluid, some small amount of said gas dissolving in said drilling fluid,but the greater proportion of said gas returning in the drilling lluidas nondissolved gas;

separating the nondissolved gas from said drilling iluid;

measuring the volume flow rate of said separated gas;

recording said flow rate measurement; repeating said process to obtainadditional measurements, and recording said further measurements;

comparing the change in measured ilow rate of said over a selectedperiod of time, to ascertain whether gas is being depleted from a gaszone, or whether additional gas is being encountered;

adjusting the density of said drilling fluid from time to time whennecessary to maintain the hydrostatic head at the lowest possible levelto provied for maximum penetration rate and maximum bit life, andminimizing the possibilty of fracturing low-pressure formations, whileat the same time preventing blowout of the well.

2. A drilling method which enables the driller to maintain drilling withav low hydrostatic head and at the same time prevent blowout whiledrilling a borehole for an oil or gas well into the earth throughgas-containing formations, whereupon gas from said formations enters theborehole, comprising:

passing drilling uid down into said borehole and circulating said fluidback to the earths surface, whereupon gas in the borehole from thesurrounding formations is returned to the earths surface by the drillingfluid, some small amount of said gas dissolving in said drilling fluid,but the greater proportion of said gas returning in the drilling iluidas nondissolved gas;

separating the nondissolved gas from said drilling lluid;

measuring the volume llow rate of said separated gas;

maintaining a record of said ilow rate and the rate of change thereof;

comparing the change in measured ilow rate of said gas over a selectedperiod of time, to ascertain Whether gas is being depleted from a gaszone, or whether additional gas is being encountered; adjusting thedensity of said drilling fluid from time to time when necessary tomaintain the hydrostatic head at the lowest possible level to providefor maximum penetration rate and maximum bit life, and minimizing thepossibility of fracturing low-pressure formations, while at the sametime preventing blow-out of the well.

References Cited UNITED STATES PATENTS 2,341,169 2/1944 Wilson et al.175-66 X 2,923,15'1 2/1960 Engle et al. 175-206 X ERNEST R. PURSER,Primary Examiner Edward M. Fletcher, Jr.

UNITED STATES PATENT oFEICE CERTIFICATE OF CORRECTION Patent No.3,498,393 March 3, 1970 Alfred Gordon West et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2 line 32 L after "been" insert shown .Column3, line 75, "p=0.52dh" shouldread p=0.052 dh Column 4, line 62, after "some" insert suchColumn 5, line 18, "fonud" should read found Column 6, line 36, after"utilized" insert with the present invention, as compared to those whichmust be used with prior art methods which is important-rather than theabsolute i value of these hydrostatic head levels. These absolutevalues, of

course, are to a great extent governed by formation properties.

Column 7, line 40, before "over" insert gas Column 8, line l, "provied"should read provide Signed and sealed this 20th day of October 1970.

(SEAL)4 Attest:

WILLIAM E. SCHUYLEE, JR.

Attesting Officer Commissioner of Patents

